lor 



each other, will require more convergence than the two correspond- 

 ent points of the centres of the concave lines, and will appear nearer, 

 whilst the two points of the centre requiring less convergence will 

 appear further ; the intermediate points from the centres to the extre- 

 mities of the two bent lines will appear gradually less distant, there- 

 fore the coalescence of the two lines bent laterally will produce the 

 illusion of a single line conspicuously concave, in a vertical plane at 

 right angles with the plane of the two separate lines. 



Having demonstrated that the semi-lenses of the stereoscope, like 

 prisms, bend laterally all the vertical lines of which the photograph 

 pictures are composed, and that these lines in the two pictures pre- 

 sent their concavity to each other, it is evident that the coalescence 

 of the two images must give the illusion of a concave image. 



The phenomenon of the lateral curvature given to vertical lines by 

 the refraction of a prism, which vertical lines, when examined with 

 two prisms, one for each eye, appear by coalescence as one line con- 

 cave in a vertical plane at right angles with the plane of the two 

 separate bent lines, can be curiously illustrated by the following ex- 

 periment : 



If, holding in each hand one prism, the two prisms having their 

 thin edges towards each other, we look at the window from the 

 opposite end of the room, we see first two windows with their vertical 

 lines bent in contrary directions ; but by inclining gradually the optical 

 axes, we can converge them until the two images coalesce, and we 

 see only one window ; as soon as they coincide, the lateral curvature 

 of the vertical lines ceases, and they are bent projectively from back 

 to front : we have then the illusion of a window concave towards the 

 room, such as it would appear reflected by a concave mirror. 



There is another phenomenon which can be noticed when looking 

 at photographic pictures in the stereoscope ; sometimes the picture 

 appears to project out and sometimes to recede from its mountings. 

 The first effect lessens the illusion, and the second renders it more 

 effectual ; therefore it is desirable to inquire how we can avoid the 

 one and ensure the other. 



We know that the distance of objects is in an inverse ratio with 

 the angle of convergence required to see them single ; also that with 

 symmetrical figures or photographic pictures, when the horizontal or 

 lateral distances of the several corresponding points is different, the 



